//
// Created by kzime on 17-8-18.
//

# include <bits/stdc++.h>
// # define MAXN
using namespace std;

inline int gn() {
    int k = 0, f = 1;
    char c = getchar();
    for(; !isdigit(c); c = getchar()) if(c == '-') f = -1;
    for(; isdigit(c); c = getchar()) k = k * 10 + c - '0';
    return k * f;
}

int zm[27], mx;
string str;

int main() {
    int n = gn(), k = gn();
    cin >> str;
    for(int i = 0; i < str.size(); i++) {
        mx = max(mx, ++zm[str[i] - 'a']);
    }
    if(mx > k) printf("NO\n");
    else printf("YES\n");

    return 0;
}
